Method for making high density nonvolatile memory

ABSTRACT

An improved method for fabricating a three dimensional monolithic memory with increased density. The method includes forming conductors preferably comprising tungsten, then filling and planarizing; above the conductors forming semiconductor elements preferably comprising two diode portions and an antifuse, then filling and planarizing; and continuing to form conductors and semiconductor elements in multiple stories of memories. The arrangement of processing steps and the choice of materials decreases aspect ratio of each memory cell, improving the reliability of gap fill and preventing etch undercut.

EXAMPLE

A detailed description of one preferred embodiment of the present invention is provided below. Due to its cylindrical shape, in this description the semiconductor element is referred to as a “beer can.” The bottom story of conductors or “wiring” will be referred to as X1, the one above it as Y2, the one above it as X3, etc. The bottom story of beer cans will be referred to as beer can 1, the one above as beer can 2, etc. This example describes creating a memory array with two stories of memory; memory arrays with more or fewer stories can be created.

Before creation of the monolithic three dimensional memory above the substrate begins, supporting circuitry, such as complementary metal oxide semiconductor (CMOS) transistors, may be created in the wafer. The steps detailed below begin with wafers processed though completion of CMOS transistors, and referred to as “the front end.” The final step is the routing layer CMP. The routing layer is are the conductive wiring connecting the CMOS transistors below, for example, to the memory cells above.

In the following description, the finished memory array includes N⁺, P⁺ and undoped polysilicon. Through all stages of deposition, photolithography, etch, and polish, all of these layers are referred to as polysilicon. In some embodiments, though, only the P⁺ silicon is actually deposited as polysilicon, while the N⁺ and undoped silicon are deposited as amorphous silicon and are crystallized into polysilicon by a final anneal, which also serves to activate dopants in the silicon.

Step 1: 200 Å TiN Deposition:

No preclean, or removal of native oxide prior to metal deposition. Deposition tool is the Applied Materials Endura, with a throughput of about 80 wafers per hour (WPH). Recipe: 200 A IMP TiN (IMP is biased sputtering which produces more conformal films than non biased sputtering). Does not need bias since no step coverage is required.

Step 2. 1500 Å Tungsten Deposition:

The tool used is the Novellus Concept One (throughput about 45 WPH). Recipe is as follows: Temperature is 445° C., pressure is 40 mTorr. Nucleation Step: 15 sccm SiH₄, 40 sccm WF₆, 1000 sccm Ar, 10000 sccm H₂, 5.5 sec. Via fill (per stage): 40 sccm WF₆, 9000 sccm Ar, 10000 sccm H₂, 6 sec. There are five stages total: one for nucleation and four for via fill, so via fill step is repeated four times.

Note: Low stress W is required. Stress: σ=1.18 E10 dynes/cm^(2.) Sheet Resistance: 1500 Å W=1.3 Ω/square.

Step 3: First Wiring Layer (X1) Photolithography:

Mask Dimensions: 0.15 μm line+0.17 μm space. Resist Coater: DNS. Scanner used is ASML/500 (40 WPH). 600 Å BARC-Shipley AR2. 4620 Å Shipley UV 135 Resist. PEB/soft bake temp=130° C./130° C. Exposure: 28 mJ/cm² (annular). Focus: 0.0 μm. DI CD Spec: Line CD: 0.16±0.02 μm. Dense to Isolated Bias: 0.04 μm. Overlay Spec:±60 nm.

If using an ASM 5500/500 DUV scanner, use an N.A. setting of 0.63 (maximum available). Use annular illumination with an inner sigma setting of 0.50 and an outer sigma of 0.80. To accomplish 0.15 mm resolution with this tool, a k₁ value of 0.37 is necessary. This is at the limit of the /500's capability. Results will be much improved with a more capable photolithography tool.

The severe proximity effects created by using the /500 are dealt with by a mask level adjustment to isolated lines. An extra 0.04 mm can be added to any side of a line that faces a space greater than 0.25 mm. This correction is process and photolithography tool specific.

Because of the heavy use of chemical mechanical polishing (CMP) and opaque thin films in the back end of the process, most alignment and overlay marks below the top surface are filled in and invisible. ASML SPM (scribe line) marks are used for layer-to-layer alignment. Achieving the necessary step height for an alignment signal depends on the differential polishing rate between the patterned and fill materials.

Note that variation in CMP can directly affect alignment. As long as the CMP process is consistent and reproducible, good overlay can be maintained. If the process shifts, or cannot be controlled, a change in overlay will be seen. Typically, CMP induces rotational, and sometimes scaling, misalignment.

Step 4: First Wiring Layer (X1) Etch:

Step 4a: Etch 1500 Å W and 200 Å TiN:

Etch tool used is LAM 9600 (35 WPH). Recipe: (Chuck Temp=45° C.). BARC: 12 mT/400 TCP/60 BP/50 BCl₃/25″. W: 12 mT/600 TCP/100 BP/60 SF₆/11 N₂/15 BCl₃/Ept (˜25″)+25% OE. TiN: 8 mT/500 TCP/80 BP/30 Cl₂/50 BCl₃/20″. The W etch rate is about 3600 Å/min.

Resist/BARC remaining after etch is about 500 Å. Oxide Gouge=˜350 Å (dense); ˜650 Å (isolated). Wall angle is 88-90°.

Step 4b: Plasma Strip:

Tool used is Gasonics (an alternative is 9600 Strip). Recipe: N2/O2 ash, 30 s, 270° C.

Step 4c: Post Strip Clean:

Tool used is Semitool. Recipe: EKC 265 Clean, 65° C., 10 min. Line CD is 0.15±0.02 μm. Dense to Isolated Bias is 25 nm. Dense lines shrink at etch by ˜15 nm while isolated lines grow by 10 nm.

Need to clear any large unstepped areas to avoid tungsten peeling.

Step 5: X1 (First Wiring Layer) Fill:

2500 Å HDP oxide deposition is performed. No preclean is required. Deposition tool can be Novellus Speed, or AMAT HDP. Silane oxide deposition conditions correspond to an etch to deposition (E/D) ratio of about 0.25 in Novellus; equivalent AMAT HDP D/S ratio=3.2.

Gap is 0.17 μm tall and 0.17 μm wide, for an aspect ratio of 1:1. AMAT HDP recommended to prevent alignment mark distortions.

Step 6: X1 CMP:

To perform CMP on HDP oxide, use Westech, with main polish rate about 2700 Å/min. AMAT Mirra can also be used. Ontrak DSS-200 with 1% NH₄OH through the brush (no HF).

Consumables used for main polish are Cabot SC-1, IC1000/SubaIV, SP1 CVD diamond conditioner disk. Consumables used for buff: Politex regular.

Recipe: Remove equivalent to 1600 Å on blanket TEOS wafer. 100 Å oxide removal in final platen slurry buff.

There is minimal W loss, some rounding of corners, smoothing of grains. Politex platen-3 buff with slurry removes approximately 100 Å from blanket wafer after main polish. Oxide dishing occurs between W lines of about 200 Å to 300 Å. Center to edge variation (oxide loss) is 200 to 300 Å.

Step 7: Diode 0 (Beer can 1) TiN Deposition:

To perform 200 Å TiN deposition, no preclean is needed. The tool used is AMAT Endura (80 WPH). Recipe is IMP TiN with bias; MOCVD or PVD TiN can be used. This layer is a barrier layer between W and polysilicon. It can be thinner, and is required to minimize leakage.

Step 8: Diode 0 (Beer can 1) Polysilicon Deposition:

Deposit 200 Å P⁺ polysilicon, 2900 Å undoped polysilicon, 800 Å N⁺ polysilicon, and 200 Å undoped polysilicon. Tool used is ASML SVG. Recipe is 540° C., 400 mTorr. Remaining recipe conditions are shown in Table 1. TABLE 1 Step Time Silane Helium 1.5% BCl₃ 1.5% PH₃  200 Å 1e21  5:00 500 sccm 700 sccm 100 sccm — P⁺ Si 2900 Å  2:22:40 500 sccm — — — undoped Si  800 Å 5e20  1:08:00 500 sccm 380 sccm — 40 sccm N⁺ Si  200 Å 10:45 500 sccm — — — undoped Si

Dopants and concentration for in-situ doping: For P⁺ dopant, use boron at 1.0×10²¹ per cm³; for N⁺ dopant, use phosphorous at 5.0×10²⁰ per cm³. Thickness variation within a wafer should be less than 2% (3 sigma); across a load, less than 3% (3 sigma); and from load to load, less than 2% (3 sigma).

600 Å of the N⁺ polysilicon thickness and the undoped polysilicon can be sacrificial for CMP polishing. At least 200 Å of N⁺ polysilicon is required for ohmic contact to subsequent TiN/W. Undoped polysilicon deposited as the last deposition eliminates autodoping, and can be removed in subsequent CMP step.

Alternatively, ion implantation can be used instead of in situ doping for the N⁺ layer. Such implantation would be done after the beer can CMP.

Step 9: Diode 0 (Beer can 1) Oxide Hard Mask Deposition:

Deposit 400 Å of oxide hard mask using a Novellus Oxide tool.

Hard mask is low temperature silane oxide. This hard mask is required if ASML/500 photolithography tool is used, but may not be necessary if a better tool is used. Hard mask improves silicon thickness uniformity after CMP, because hard mask deposition is more uniform than fill deposition.

Step 10: Diode 0(Beer can 1) Photolithography:

Use DNS resist coater and ASML/500 scanner or ASML/700 scanner. Deposit 900 Å BARC (or, alternately, DARC) and 4270 Å UV 135 resist. Perform PEB and soft bake at 130° C./130° C. Exposure is 20 mJ/cm². Focus: 0.0 μm.

The beer can ideal CD is 0.15±0.02 μm. Overlay specification is ±60 nm.

If the beer can mask is to be printed using ASML/500, it should be sized up to 0.17 μm and printed with about a 20 nm positive bias. If an ASML/700 scanner or better will be available, the mask may be printed with better fidelity.

Other problem associated with using /500 for 0.15 μm posts are: low resist thickness (about 3500 Å resist after develop) and poor profile warranting a hard mask at etch.

Misalignment can be tolerated. At nominal dimensions for beer cans and W conductors, contact area varies with misalignment as shown in Table 2: TABLE 2 Linear Misalignment Contacted Area  +/−0 nm 100%  +/−50 nm 70%  +/−75 nm 50% +/−100 nm 30% +/−125 nm 17%

In the case of the wiring layers: X1, Y2, . . . , X9, the polishing rate for tungsten is near zero so the CMP selectivity of tungsten to oxide is high. This makes it relatively easy to make a useable alignment mark. ASML specifies a target step height of 1200 Å. Experience has shown that for this process, any step height greater than 200 Å is sufficient to get good alignment. Between 40 Å and 200 Å, alignment may still work, but it is not reliable. Values greater than 200 Å are typically achieved for the wiring layers.

Step 11: Diode 0 (Beer can 1) Etch:

Etch the following layers: 900 Å BARC, 400 Å Oxide, 200 Å undoped polysilicon, 800 Å N⁺ polysilicon, 2900 Å undoped polysilicon, 200 Å P⁺ polysilicon and 200 Å TiN. The etch tool used is 9400 DFM (44 wph for 4 chamber tool).

The following etch recipe is used (cathode Temp: 60° C.): For BARC, 5 mT/250 TCP/−200 BV/100 CF₄/Ept. For the hard mask, 5 mT/250 TCP/−200 BV/100 CF₄/20″. Polysilicon main etch (ME): 10 mT/350 TCP/90 BP/40 Cl₂/150 HBr/15 He O₂/Ept(˜100 s)+5% OE. Polysilicon over etch (OE): 80 mT/400 TCP/60 BP/100 HBr/200 He/19 He O₂/45″. TiN: 5 mT/250 TCP/120 BP/10 Cl₂/50 HBr/50 He/30″.

To perform the plasma resist strip, use the Novellus Iridia. The recipe is ˜2.5% CF₄ in O₂, 1375 W MW, 50 W RF, 40° C., 105 s. 100% O₂, 0 W MW, 420 W RF, 40° C., 60 s.

The solvent clean tool is Semitool., and the recipe is EKC 265 Clean—10 minutes. The beer can CD is 0.21±0.03 μm. The CD is higher due to mask biasing. The ideal can CD is 0.17±0.02 μm. CD Bias: ˜20 nm. relative to photolithography.

Gouge into W during OE is <100 Å, and gouge into oxide during over etch is<100 Å.

Polysilicon OE step is selective to TiN. CF4 containing ash with bias in Iridia helps clean up polymer without HF dip.

Step 12: Diode 0 (Beer can 1) Fill:

For 4000 Å HDP Oxide Deposition (3 chambers; 60 WPH), use Novellus Speed or AMAT HDP. In the case of Novellus deposition, the E/D ratio=0.25; equivalent AMAT HDP D/S ratio=3.2.

Thickness should be 4100±200 Å. AMAT HDP is recommended since Novellus HDP fill causes asymmetry and thereby affecting alignment at photo.

Step 13: Diode 0 (Beer can 1) CMP:

To CMP the HDP oxide, tools and consumables are the same as for the conductor polish in Step 6. Oxide removal target is 700 Å. Oxide hard mask must be removed from beer can structures. It is acceptable to leave undoped polysilicon on beer cans after polish. It is acceptable to leave unpolished oxide on other structures. After polish, the beer can must have a minimum of 200 Å of P⁺ polysilicon remaining, such that total stack height is 3100 to 4300 Å after polish.

Polish times are very short for 700 Å target. Ramp-up plus buff steps remove approximately 250 Å. Main polish step is typically 9 to 10 seconds. The buff step, if used removes approximately 100 Å after main polish. Center to edge oxide loss variation is about 100 to 200 Å.

Step 14: Beer Can 1 Antifuse Growth:

18 Å of antifuse oxide must be grown. Perform preclean first: 100:1 HF dip; 45 s. Antifuse oxide growth is performed using an AG Associates RTP tool. Recipe is 650° C.; 60 s; Gases: 5 L O₂ and 5 L N_(2.) Thickness of the antifuse is 18±1 Å. Within wafer thickness uniformity should be less than 3% (1 sigma).

Maximum queue time between preclean and RTO is four hours. Queue time between RTO and Y2 TiN deposition appears not to be critical. AMAT RTO is an alternate tool.

RTO conditions will be adjusted based on diode e-test results (unprogrammed leakage current and time to breakdown).

Step 15: 200 Å TiN Deposition, Second Wiring Layer (Y2):

Tool used is Endura MOCVD or ULVAC PVD can be used. Recipe: IMP TiN without bias. The TiN deposition step should not have any pre-sputter clean or bias in order to protect the antifuse oxide. PVD or MOCVD TiN deposition can be used.

Step 16: 1500 Å W Deposition, Second Wiring Layer (Y2):

The Novellus W tool can be used. The recipe should be the same as X1 W deposition in Step 2 but for deposition time. Deposition time adjusted to target 1500 Å. TiN deposited without bias may make the W nucleation process sluggish, which is referred to as an incubation time, making the resulting W thickness lower compared to X1 step. W deposition on PVD or MOCVD TiN may behave differently.

Step 17: Y2 Photolithography—Second Wiring Layer Masking:

Photolithography conditions same as X1 photolithography conditions of Step 3 except dose of 27.5 mJ/cm². CD specification is 0.16±0.02 μm, and overlay specification is ±60 nm.

Step 18: Y2 Etch—Second Wiring Layer Etch:

1500 Å W and 200 Å TiN must be etched. Etch conditions are the same as X1 etch conditions described in Step 4. CD spec is 0.15±0.02 μm. Gouge into beer can 1 polysilicon is about 1400 Å.

Step 19: Y2 Fill:

2500 Å HDP oxide fill. Fill conditions are the same as for X1, described in Step 5.

Step 20: Y2 CMP:

CMP conditions same as for X1, describe in Step 6.

Step 21: Diode 1 (Beer Can 2) TiN Deposition.

The process to deposit 200 Å of TiN is the same as for beer can 1 TiN deposition, described in Step 7.

Step 22: Diode 1 (Upside down diode—Beer can 2) Polysilicon Deposition:

To deposit 200 Å N⁺ polysilicon, 2900 Å undoped polysilicon, 700 Å P⁺ silicon, and 300 Å undoped silicon, use the ASML SVG tool. Recipe is 540° C., 400 mTorr, with remaining conditions detailed in Table 3: TABLE 3 Step Time Silane Helium 1.5% BCl₃ 1.5% PH₃  200 Å 5e20 19:30 500 sccm 380 sccm — 40 sccm N⁺ Si 2900 Å  2:22:40 500 sccm — — — undoped Si  700 Å 1e21 15:00 500 sccm 700 sccm 100 sccm — P⁺ Si  300 Å 15:45 500 sccm — — — undoped Si

Dopant and concentration for in-situ doping, for N⁺ should be phosphorous at 5.0 E20 per cm³. For P⁺, use boron at 1.0 E21 per cm³. Thickness variation is the same as for beer can 1, as described in Step 8.

500 Å of the P⁺ thickness and undoped polysilicon is sacrificial for CMP polishing. About 200 Å of N⁺ and P⁺ polysilicon is required for Ohmic contact to the underlying TiN/W. The undoped cap eliminates memory effect during deposition.

Alternatively, ion implantation can be used instead of in situ doping for the N⁺ layer. Such implantation would be done after the beer can CMP.

Step 23: Diode 1 (Beer can 2) Hard Mask Deposition:

To deposit a 400 Å hard mask oxide, the deposition conditions are the same as for beer can 1 hard mask, described in Step 9. This hard mask is needed to prevent poor photolithographic profile caused by scanner limitations from being transferred to the diode at etch. The hard mask also helps CMP.

Step 24: Diode 1 (Beer can 2) Photolithography:

Beer can 2 photolithography is the same as beer can 1 photolithography, as described in Step 10.

Step 25: Beer Can 2 Etch:

Beer can 2 etch is the same as beer can 1 etch, as described in Step 11.

Step 26: BC2 (Beer can 2) Fill:

Beer can 2 fill is the same as beer can 1 fill, as described in Step 12.

Step 27: Beer can 2 CMP:

Beer can 2 CMP is the same as beer can 1 CMP, as described in Step 13.

Step 28: Beer can 2 Antifuse Growth:

To grow 18 Å of antifuse oxide, perform a preclean: 100:1 HF, 45 s. Grow the antifuse using the AG Associates RTP. The recipe is 650° C., 5 l O₂, 5 l N₂, 60 s. In beer can 2, antifuse oxide is grown on P⁺ polysilicon as opposed to on N⁺ polysilicon in the case of beer can 1.

RTO conditions will be adjusted based on diode e-test results (unprogrammed leakage current and time to breakdown).

Step 29: X3 TiN 1 Deposition:

To deposit 300 Å of TiN, no preclean is required. The tool and recipe for this TiN deposition are the same as for the Y2 TiN deposition, described in Step 15, except for the deposition time. The deposition time is compensated for increased thickness

Plasma damage prevention is critical during TiN deposition in order to protect the antifuse. TiN is needed to protect the antifuse during zia etch and cleans. 300 Å of TiN is needed to protect the antifuse from being exposed during the TiN deposition prior to X3 W deposition.

Step 30: Zia 1 Photolithography:

Use 4620 Å of UV135 resist, and 600 Å of BARC. The scanner is the ASML/500 (NA/sigma: 0.48/0.36). Alternatively ASML/700 can be used instead. Expose conditions are 31 mJ/cm²/−0.1 μm Focus. CD should be 0.21±0.02 μm. Overlay is ±75 nm. 0.21 μm wide zias are targeted. Three different zia lengths exist in the cell. Different lengths can be used depending on how many memory levels are to be contacted.

Step 31: Zia 1 Etch:

Etch 300 Å TiN and about 14000 Å oxide+25% OE. Etch tool is Centura MxP-eMax. Etch recipe for BARC/TiN Etch is Ar/CF₄/CHF₃ chemistry. For the oxide etch the recipe is Ar/C₄F₆/O₂ chemistry.

The CD specification is 0.21±0.025 μm (this is the top CD.) CD bias<10 nm (relative to photolithography for top CD.)

Regarding selectivity, resist loss during TiN etch is 500-600 Å. Oxide to resist selectivity is 6:1. Oxide to W selectivity is 100:1, so there is no noticeable W loss.

The wall angle is 88±1°. For resist strip, use Gasonics—O₂/N₂ ash; 270° C. Solvent clean is performed using Semitool 10 min EKC Clean.

Note BARC may not be needed on top of TiN. TEL DRM is an alternate tool for this process.

Zias are rectangular. Zias of three different lengths are present in the mask. Selectivity to W during zia etch is very good.

Step 32: X3 TiN 2 Deposition:

Perform the second TiN (200 Å) deposition at X3 using the AMAT Endura tool. The recipe: IMP TiN or, alternatively, MOCVD TiN.

Pre-sputter is equivalent to an oxide loss of 300 Å on a blanket oxide wafer, or a loss of 120 Å of TiN on a blanket TiN wafer.

Pre-sputter step is critical since it serves two competing purposes. That is, W surface at the bottom of the zia should be cleaned of native oxide while ensuring that the antifuse oxide is not exposed or damaged.

The total TiN thickness (TiN1+TiN2−pre-sputter) on the wafer is 350-400 Å at the end of this step.

Second TiN thickness can be reduced if MOCVD is used.

Step 33: X3 W Deposition:

For 2000 Å W Deposition, use Novellus CVD W tool. No preclean. Recipe is the same as X1 or Y2 W deposition except for time, which is adjusted for thickness target.

Note that W thickness is 2000 Å. This is required to fill 0.35 μm wide zias. If zia size is reduced to below 0.3 μm, W thickness can be reduced to 1500 Å, as in the case of X1 and Y2.

Step 34: X3 Photolithography:

Photolithography conditions and specifications are the same as that of Y2 photolithography steps, described in Step 17. CD specification is 0.16±0.02 μm; overlay specification is ±60 nm.

Step 35: X3 Etch: Etch 2000 Å of W and 400 Å of TiN:

Etch and clean conditions and specification are the same as that of X1 and Y2 etch steps. Etch times should be adjusted in W and TiN etch steps to account for increased thickness relative to X1 and Y2. Need to verify if resist thickness is sufficient to tolerate longer etch relative to X1 and Y.

Step 36: X3 Fill: Deposit 3000 Å of HDP Fill Oxide:

Same as X1 and Y2 fill except for thickness. Increased oxide thickness to account for increased W thickness.

Step 37: RTA: Final Anneal

Use the AG Associates RTA tool. Recipe is 770° C., 60 s; 10 liters Ar flow. The purpose is to crystallize silicon and activate dopant.

Step 38: X3 CMP:

Blanket wafer CMP removal target is 1700 Å. Other conditions are the same as X1 and Y2 CMP, described in Step 6.

Step 39: X3 Cap Oxide Deposition:

Deposit 5000 Å of Silane Oxide using Novellus tool. Recipe is Silane oxide. This need not be HDP oxide since it is a cap oxide on a CMPed surface. Can be AMAT oxide or Novellus.

Step 40: Final Zia Photolithography:

The final zia makes contact between top metal and memory layers.

Use ASML/500 (NA/sigma: 0.48/0.36). Photolithography conditions are the same as Z1 except dose of 30 mJ/cm². CD is 0.21±0.02 μm; overlay is ±75 nm.

Step 41: Final Zia Etch:

Etch 600 Å of BARC, 5000 Å of oxide+25% OE. Use Centura MXP eMax tool. Etch chemistry can be the same as Zia 1 etch described in Step 31. TiN etch step should be removed. Oxide etch time should be adjusted to target 8000 Å of oxide removal. Ash and solvent clean: same as zia 1. described in Step 31. CD Spec is 0.21±0.025 μm. Etch is straightforward and simple since the stack height is short and W is a good etch stop. Same recipe used for etching zia 1 can be used after removing TiN etch step and altering time to account for thickness difference.

Step 42: Final Zia TiN Deposition:

Use AMAT Endura tool. Recipe is 200 Å IMP TiN; MOCVD TiN can be used. Recipe conditions can be the same as the TiN deposition condition used for second TiN deposition at X3 deposition.

Step 44: Final Zia W Deposition:

For 2000 Å W deposition, use the Novellus W tool. The recipe is the same as for the X3 W deposition described in Step 33. Note that W thickness is 2000 Å. This is required to fill 0.35 μm zias. If zia size is reduced, W thickness can be reduced accordingly.

Step 45: Final Zia W CMP:

Use IPEC 472. Consumables are Rodel MSW 1500 (KIO₃—alumina) slurry, Politex main polish pad, no buff. Polish rate is about 3000 Å/min. Polish time is 90 seconds. Poor oxide selectivity; oxide loss is approximately 1000 to 1500 Å. Alternately, any via or contact W polishing recipes can be used instead.

Step 46: Top Metal Deposition:

Deposit 150 Å Ti, 4000 Å Al, 300 Å TiN. Use AMAT Endura tool. Recipe: Deposit 150 Å PVD Ti; Presputter is equivalent to 50 Å of oxide. Deposit 4000 Å PVD Al at 200° C. Deposit 300 Å PVD TiW. Any top metal stack conditions can be used.

Step 47: Top Metal Photolithography:

Use ASM11. Resist is JSR iX715 DM7 (1.2 μm). BARC: AR2. Photolithography conditions: 220 mJ/cm²/0.0 μm. CD is 1.0 um±0.1 μm.

Step 48: Top Metal Etch:

Tool: LAM 9600 PTX. Etch recipe is Cl₂/BCl₃/SF₆ BARC and TiW etch. Cl₂/BCl₃ Al etch, OE, and Ti etch.

Use standard plasma strip conditions. Solvent Clean tool is Semitool, 20 min EKC-265, 65° C.

Step 49: Alloy:

Use VTR Furnace. Recipe: Alloy at 400° C. and atmospheric pressure for 30 minutes in N₂ with about 5% H₂.

Periodic backside removal was found to be necessary to process the wafers through all the photo and etch steps. Stress measurements indicate linear increase in stress with layer number and stress relief with backside film removal. Backside polysilicon removal after every fourth polysilicon stack deposition would prevent any stress related processing problems and ensure sufficient process margin. Another option is to add backside polysilicon removal after every stack deposition starting with P⁺ polysilicon (as in the case of beer can 1.) This would also alleviate defect concerns.

More backside etches may be necessary to meet particle performance.

Care must be taken in setting up the photolithography job such that photolithographic patterning leaves no part of the wafer unexposed (such as wafer scribe region). W peeling is typically seen in the alignment mark region and in big pads.

Thick oxide on W and excessive time at high temperature (greater than 700° C.) exacerbate W peeling problem. 

1. A three-dimensional memory cell of a first story of an electronic device, the memory cell comprising: a) a first conductor extending in a first direction; b) a first semiconductor element over the first conductor, the first semiconductor element comprising: i) a first heavily doped semiconductor layer of a first conductivity type; ii) a second heavily doped semiconductor layer of a second conductivity type, the second semiconductor layer disposed above the first semiconductor layer; iii) a third lightly doped semiconductor layer interposed between the first and second semiconductor layers; iv) an antifuse layer disposed above the second semiconductor layer or below the first semiconductor layer; and c) a second conductor over the first conductor, the second conductor extending in a second direction different from the first direction.
 2. The memory cell of claim 1 wherein the antifuse layer is disposed above the second semiconductor layer.
 3. The memory cell of claim 2 wherein the antifuse layer is a dielectric layer.
 4. The memory cell off claim 3 wherein the antifuse layer comprises silicon dioxide.
 5. The memory cell of claim 2 wherein the conductors comprise tungsten.
 6. The memory cell of claim 2 wherein the cell is disposed above and not in contact with a monocrystalline semiconductor substrate.
 7. The memory cell of claim 1 further comprising a second semiconductor element over the second conductor.
 8. The memory cell of claim 7 further comprising a third conductor formed over the second semiconductor element.
 9. The memory cell of claim 1 wherein the first semiconductor element comprises polysilicon.
 10. The memory cell of claim 1 wherein the first semiconductor element comprises germanium.
 11. The memory cell of claim 1 wherein the first semiconductor element comprises silicon-germanium.
 12. The memory cell of claim 1 wherein the first semiconductor element comprises silicon-germanium-carbon.
 13. The memory cell of claim 1 wherein the memory cell is a portion of a monolithic three dimensional memory array.
 14. A monolithic three dimensional memory array comprising: a) a first story of cells, the first story comprising: i) a first conductor extending in a first direction; ii) a first semiconductor element comprising an antifuse layer formed above the first conductor; iv) a second conductor formed above the semiconductor element, wherein the antifuse layer is in contact with the second conductor; and b) at least a second story of cells formed above the first story.
 15. The memory array of claim 14 wherein the antifuse layer is a dielectric layer.
 16. The memory array of claim 15 wherein the antifuse layer is a silicon dioxide layer.
 17. The memory array of claim 14 wherein the semiconductor element comprises a first heavily doped semiconductor layer, and wherein the antifuse layer is formed on and in contact with the first heavily doped semiconductor layer.
 18. The memory array of claim 14 wherein the conductors comprise tungsten.
 19. The memory array of claim 14 wherein the first semiconductor element comprises germanium.
 20. The memory array of claim 14 wherein the first semiconductor element comprises silicon-germanium.
 21. The memory array of claim 14 wherein the first semiconductor element comprises silicon-germanium-carbon.
 22. The memory array of claim 14 wherein the second conductors of the first story of cells serve as lower conductors of an overlying, second story of cells. 